The use of lasers is gaining interest for transmitting information optically, either for long or short distances on earth or in space. Such devices are also of interest for optical computers. Optical transmissions have potential advantages of great speed, because of the high speed of light and the high optical frequencies compared to electronic based systems.
A laser diode provides a high efficiency, compact means for converting electronic signals to optical signals. For example an amplitude modulation of the driver current for the diode generates a frequency modulation in the optical frequency of the laser. Such a simple modulation is not very useful for conveying complex information, e.g. binary information. Application of small signal pulses to a diode ordinarily results in non-linear responses in the diode because response varies with the modulation frequency. A further problem is the relative low power output of a laser diode.
Monolithic laser diode arrays have the potential for providing quasi-continuous-wave output power greater than 50 W, compared with 3 mW for a single diode in typical single-mode operation. U.S. patent application Ser. No. 232,074 now U.S. Pat. No. 4,927,263 filed Aug. 15, 1988 (de Groot et al) of the present assignee discloses use of an array of laser diodes incoherently for position and velocity imaging of a target. Phase locking of small incoherent arrays has been demonstrated using an external cavity and optical feedback to force mutual coherence between the diodes, as disclosed, for example, in "Mode control of an array of Alga-As lasers using a spatial filter in a Talbot cavity", by F. X. D'Amato, E. T. Siebert and C. Roychoudhuri, SPIE Proc. 1043, Pg. 100 (1989).
Ordinarily the addition of an external cavity to the diode array considerably complicates the modulation problem, because the external cavity tends to make stationary many of the source characteristics that one might wish to modulate in order to encode information. In particular, the frequency locking behavior of an external cavity precludes frequency modulation except in those special cases where the modulation spectrum matches the mode structure of the cavity.